Network equipment manufacturers assign 48-bit identifiers called media access control (MAC) addresses to network devices they make. MAC addresses are allocated to manufacturers in blocks by the Institute of Electrical and Electronics Engineers, Inc. (IEEE) Registration Authority. All MAC addresses in an allocated block include the same three-byte “organizationally unique identifier” (OUI) reserved for the manufacturer.
The allocation of MAC addresses with OUIs is intended to encourage interoperability of network devices. If network devices made by different manufacturers applied in the same network are to communicate unambiguously through exchange of MAC-addressed frames, each network device must be assigned a MAC address that is unique. And since it is not known a priori in which network a device will be applied, each manufacturer must assign to a network device a MAC address which is unique among not only its own manufactured devices, but unique among network devices made by other manufacturers as well. Such global uniqueness is guaranteed by allocating MAC addresses having OUIs.
Although most MAC addresses are used for communication between network devices, as outlined above, use of MAC addresses exclusively within a single network device is becoming more common. One such “internal only” use of MAC addresses arises in the context of exceptional frame forwarding within a LAN switch. Generally speaking, a LAN switch is a multi-port LAN device which interconnects LAN end-stations residing in different LAN broadcast domains, or LAN segments, through different ports on the switch. Such a switch typically forwards conventional frames received from a source end-station over a first port to a destination end-station over a second port based on MAC addressing information encoded in the frame and the switch's knowledge of the port through which the destination end-station can be reached. Such a switch may, however, depart from convention when forwarding certain exceptional frames. To perfect exceptional frame forwarding, MAC addresses may be temporarily assigned to interfaces or ports of the switch and encoded in exceptional frames upon internal transmission to cause such frames to be captured by such interfaces or ports. Such temporarily assigned and encoded MAC addresses are never “seen” on an external LAN transmission medium; they are only “seen” internally.
Because MAC addresses applied in “internal only” uses are never “seen” outside the network device to which they are assigned, maintaining their global uniqueness among network equipment manufacturers to avoid addressing ambiguities is not necessary. Manufacturers nonetheless often assign globally unique MAC addresses (i.e. allocated by the IEEE Registration Authority and having the manufacturer's globally unique OUI) for “internal use only” applications in network devices, reducing the number of globally unique MAC addresses remaining in the available MAC address pool unnecessarily.
As described in Internet Engineering Task Force Requests for Comment (IETF RFCs) 1062, 1122 and 1700, it is known in the context of Internet Protocol (IP) addressing to reserve the network number “127” for a host “loopback” function for use on any IP host such that any IP datagram having a network “127” address loops-back inside the host and never appears on any IP network. However, IP host loopback addressing is directed to a Layer 3 (i.e. routing) protocol. Moreover, there is no suggestion to assign loopback addresses to switching devices or applying such addresses in special processing of inbound datagrams.
As described in RFC 2464, it is also known to locally administer MAC addresses by setting a “U/L” bit at a particular bit position in the 48-bit address. However, locally administered MAC addresses are not reserved for “internal only” uses and may be encoded in frames transmitted on external LAN media.
Accordingly, it would be desirable to implement a reservation, allocation, assignment and application protocol for networking addresses, such as MAC addresses, which takes advantage of the fact that addresses used exclusively in internal applications would not cause addressing ambiguities even if assigned to a plurality of devices in the same network.